1. Field of the Invention
The present invention relates to a polyester film and a photosensitive material using the same, and more particularly to a copolymerized polyester film which has excellent transparency, water absorption and mechanical properties and a photosensitive material using the polyester film as a base layer or a cover layer thereof.
2. Description of the Prior Art
There are a sheet type material such as an X-ray film, an engraving film or a cut film and a roll type material (roll film) as a photosensitive material. A typical roll film is a color or black-and-white negative film enclosed in a cartridge with a width of 35 mm or smaller than 35 mm and used for a camera.
A photosensitive material is generally produced by coating at least one photosensitive layer on a plastic base film. A fibrous polymer represented by triacetyl cellulose (hereinafter, referred to as "TAC") and a polyester polymer represented by polyethylene terephthalate (hereinafter, referred to as "PET") are used for this plastic base film, as shown in, for example, "Plastic Films, Third edition, page 52, J. H. Briston, (1989)" and "The Science and Technology of Polymer Films, Volume II, page 588-592, Orville J. Sweeting, (1971)".
TAC film is made by solution film formation using methylene chloride as a main solvent. TAC film has a small optical anisotropy, and excellent transparency and flatness. Further, since TAC film has an appropriate water absorption property, it indicates an excellent curl-extinguishing property after processing. Namely, although a curl is generated in the film by keeping the film in a state of a roll formation for a certain period of time, the manoeuverability of the molecule chain in the film increases by the water absorption in processing of the film, and the molecule chain which has been fixed in the state of roll formation is rearranged by the enhanced motion, thereby extinguishing the curl.
In contrast, in a photosensitive material using a film which does not have such a curl-extinguishing property, if the material is used in the form of a roll, troubles such as generation of scratches, blooming of focus, or jamming in carrier process occur in a printing process wherein a picture image is formed on a photographic paper after processing. Therefore, TAC film, which does not cause such troubles, has been broadly used as a photosensitive material.
However, recently the requirements for photosensitive materials have been diversely extended. Particularly, properties capable of following a high-speed unwinding of a roll film in photographing, a high magnification in printing and slide projection, and a property applicable to a miniaturized camera have been strongly required. Therefore, characteristics such as high mechanical properties and dimensional stability have been required for a base film of a photosensitive material as well as a thinner film has been required as the base film.
A biaxially oriented PET film has excellent transparency, mechanical properties and dimensional stability, and it has been used instead of a TAC film in the fields of a micro film calling for a thin film and a sensitive film for printing calling for a strict dimensional stability. In a PET film, however, because its water absorption property is poor, it cannot have a high curl-extinguishing property such as one of TAC film. Therefore, use of a PET film is restricted because the handling ability thereof after processing is not good, although it has excellent properties as mentioned above.
Some attempts giving a water absorption property to a PET film by improving a PET polymer have been proposed. For example, JP-A-HEI 1-244446 discloses a biaxially oriented film composed of a polymer which is prepared by copolymerizing PET with 5-sodium sulfoisophthalic acid and adipic acid. However, since the water absorption speed thereof is lower than that of a TAC film, a sufficient curl-extinguishing property cannot be obtained. U.S. Pat. Nos. 4,217,441 and 4,241,170 disclose biaxially oriented films wherein polyethylene glycol and 5-sodium sulfoisophthalic acid are copolymerized with PET. However, the object of these proposals is merely to give a permeability to water vapor to the films, and the films are not subjected to a photographic roll film. Therefore, although these films have some water absorption properties, the curl-extinguishing properties thereof are lower than that of a TAC film. Moreover, the transparencies of the films deteriorate by the aging properties, and the Young's moduluses thereof after water absorption greatly decrease.